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Cell Wall & Membrane

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Let’s begin our journey somewhere pleasant: the beach!

I thought I could relax for the whole day… until I saw how crowded it was. Just too many people! Nevertheless I must busily search for my first pair of organelles. The cell wall should look like a rigid, rectangular, and green frame. The plasma membrane is a thin layer just below it. Did you find them?

<Scroll to bottom for correct location…>

Under a microscope, cell walls are easy to observe. The rectangular boundaries are clearly visible.

Ah, first i must introduce you to the cell wall and the plasma membrane. They are both outermost components of a plant cell that are crucial to maintaining its shape and structure, as well as regulating the movement of molecules in and out of the cell.

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[Cell Wall]

Cell wall structure

Structure

The cell wall is composed of three major regions:

middle lamella: outer layer that borders adjacent cells and is made of protein and pectin

secondary wall: the most rigid part, a layer beneath the primary wall, that resists compression and is mostly made of cellulose and lignin

Function

structural support, maintaining shape: the cell wall can resist tension and compression, pushing on the cytoplasm within the cell.

regulating cell growth: the cell only grows where the secondary wall has not formed, and its growth direction is determined by the arrangement of cellulose microfribils

physical barrier to larger molecules and possible pathogens

metabolism: some enzymes are embedded into the cell wall

The cell wall minimizes outer contraction despite plasmolysis.

Elodea cells in a hypertonic solution.

[Plasma Membrane]

Structure

Just beneath the cell wall lies the plasma membrane, a lipid bilayer composed of phospholipids. Phospholipids are amphiphilic, meaning that they contain both hydrophilic and hydrophobic sections; their hydrophilic heads face outward and their hydrophobic tails face inward. In a phospholipid, two fatty acid chains are attached to a complex bondage of glycerol, phosphate, and choline.

The plasma membrane must also provide enough surface area for the cell to exchange waste for necessary molecules. It maintains a spherical or tubular shape enclosing the cytoplasm to maximize the surface area to volume ratio.

– The membrane is embedded with…

integral proteins: traversing the whole membrane

peripheral proteins: attached to the surface of the membrane

enzymes: specialized proteins that aid metabolism as catalysts

glycolipids: lipids attached to a short carbohydrate chain

glycoproteins: proteins attached to a short carbohydrate chain

cholesterol: a type of steroid

Function

Unlike the cell wall, the membrane is a fluid mosaic. Because phospholipids and proteins are constantly changing places, the plasma membrane does not have a rigid shape or structure. As a thin, flexible layer, it is important for the regulation of molecular exchange and electrochemical reception.

– cholesterol is an element that maintains fluidity of the membrane regardless of its environment’s temperature. At low temperatures, it prevents phospholipids from packing together, whereas at room temperature, it gives more rigidity by reducing the movement of phospholipids.

– the function of proteins in the membrane can be categorized:

transport: there are two types of transport, passive and active. Passive transport such as diffusion, facilitated diffusion, and osmosis do not require energy input; molecules flow from high to low concentration. Ion channels, for example, contain a hydrophilic pathway through the center to allow the passage of polar molecules. By contrast, active transport requires energy to be spent on mechanisms such as carrier proteins and transport proteins . The sodium potassium pump is a common example of active transport.

enzymatic activity: enzymes in the membrane convert substrates to new products. Many of them work closely with the smooth endoplasmic reticulum.

signal transduction: chemical signals, such as hormones or neurotransmitters, are recognized by receptor proteins. Cells also respond to electric signals using receptor proteins.

cell to cell recognition/joining: membrane proteins allow cells to interact with each other. The carbohydrates on glycoproteins serve to identify the cell, and other proteins may create a juncture between two cooperating cells.

attachment to the cytoskeleton and extracellular matrix

The plasma membrane engages in passive and active transport.

Extras

Click here to watch an interactive animation about cellular transport.

Click here to watch McGraw Hill’s animation of the sodium potassium pump, also called the Na +/K+-ATPase. This regulation of a Na+ and K+ concentration gradient is crucial to transportation, resting potential, cell volume, and molecular signals.